At the present time, electrical power is generally generated and distributed as alternating current (AC) power since voltage can be readily changed through use of transformers; allowing very high voltage and relatively low current for power transmission over large distances with reduced ohmic losses. However, many electrical and electronic devices must operate from a more or less constant voltage and conversion from AC power to direct current (DC) power is required. In some cases, it has been found effective to distribute power over relatively short distances as DC power. Such DC distribution arrangements are sometimes referred to as a DC nanogrid. Also, in recent years, environmental concerns have caused increased interest in so-called renewable energy resources such as solar power which is generated and stored as DC power but, when generated on a large scale, must be converted to AC power for transmission and distribution over the existing power distribution grid. Accordingly, power converters have been developed to not only convert AC power to DC power and vice-versa but also to transfer power bi-directionally.
To obtain maximum efficiency in power converters, various switching topologies are generally used rather than analog circuits, particularly to obtain regulated DC power or to obtain AC power of a desired frequency from DC power. Switching is generally performed at relatively high frequencies to accommodate large load transients and allow smaller and less costly magnetic components (e.g. transformers, inductors and the like) to be used. However, high frequency switching causes so-called electromagnetic interference (EMI) noise which must be removed by filtering. AC/DC converters may also exhibit leakage current at both the line frequency and double the line frequency as well as at the switching frequency, particularly when so-called unipolar pulse width modulation (UM), also referred to as three-level modulation, is used to drive the switches. Such leakage currents can present a safety issue as well as generating EMI noise and causing aging of components and apparatus connected to the power converter. Photovoltaic (PV) cells of which solar power generation systems are comprised are particularly subject to damage from leakage currents.
Most of the known power converter topologies for commercial solar power generation include a galvanic transformer that provides isolation from the power distribution grid. Isolation through use of a transformer not only ensures safety under most circumstances but also reduces EMI noise and step-up or step down of the DC power voltages developed by the PV cell arrays. However, if the transformer is operated at the low line frequency, the transformer must be of large size, weight and cost to deliver significant power. If the transformer is operated at a high switching frequency, more switching devices and conversion stages are required, significantly compromising overall system efficiency, cost, performance and reliability.
Accordingly, converter topologies that do not include a transformer have recently received substantial attention. Many such topologies are relatively simple and are of high reliability and efficiency for low voltage power ratings of 10 kW or less although they do not generally provide isolation. (Isolation is not required in the power distribution grid standards in many countries.) Among such topologies, the full-bridge topology is well-accepted in single phase power conversion applications such as are generally used for solar power generation using PV cell arrays.
Full bridge power converters are used as the rectifier to convert AC power for use by DC loads or power storage in batteries and for bi-directional power transfer, particularly where conversion between AC power and DC power is desired, such as in electric vehicles and small-scale AC/DC power distribution systems such as may be used in residences and vehicles such as aircraft or water-borne vessels and so-called DC nanogrids including both DC power sources and AC and/or DC loads and where power may be distributed over short distances as DC power.
Unipolar (so-called because the upper and lower switches switch between zero volts and +V/2 or −V/2, respectively) pulse width modulation (PWM), also referred to as three-level modulation, employs two sinusoidal reference signals of opposite signs to modulate switching pulse width in each phase leg and is usually used to operate full-bridge power converters due to the low differential mode (DM) noise generated which, in turn, allows use of an AC filter of relatively smaller size and cost. However, unipolar modulation generates a large high frequency leakage current flowing to ground through parasitic capacitance on the DC side of a full-bridge converter that can have a profound effect on the aging of components connected to the DC side of the converter, particularly PV cells as alluded to above and can compromise safety. The parasitic capacitance is of particularly large value and effects of leakage currents are particularly aggravated when the DC side is connected to an array of PV cells that may present an effective capacitor plate area that may be measured in acres. Therefore, such leakage current must be suppressed and several arrangements for doing so have been proposed which have proven effective to a greater or lesser degree. However, the leakage current suppression arrangements proposed to date have included additional active power switches which increase cost and produce additional switching losses as well as reducing reliability and requiring complex driving circuits. Further, some proposed leakage current suppression arrangements preclude bi-directional converter operation.